Toxin‐antitoxin systems and their role(s) in persistence

Abstract

During the last decade, efforts to understand the molecular mechanism(s) that underlie bacterial persistence have accelerated substantially. In spite of these efforts, the molecular activities that lead to the persister phenotype remain largely unknown. Persister cells are characterized by an upregulation of genes known as toxin‐antitoxin (TA) pairs, with the toxin mqsR being the most highly upregulated gene in E. coli persisters. During normal cell growth, MqsR and its antitoxin, MqsA, form a non‐toxic complex. However, during stress, MqsA is degraded allowing MqsR to induce growth arrest via mRNA cleavage. We previously reported that the mqsRA TA pair is unique because of the unusual features of MqsA. Now, our most recent work reveals that MqsR also has exclusive properties. Using X‐ray crystallography, mutagenesis, in vitro RNA cleavage assays, EMSAs and ITC, we show that, unlike all other TA systems, the MqsR‐MqsA complex cannot bind DNA. Instead, MqsR functions solely as a de‐repressor, further promoting multidrug tolerance through its ability to disrupt MqsA‐mediated repression of persistence‐related genes. Furthermore, using in vitro studies, we discovered that MqsR is a ‘hybrid’ endoribonuclease (RNase), having characteristics of both RelE‐like and microbial like RNases. Recent work has defined the mechanism of mRNA cleavage and the molecular basis of MqsA neutralization of MqsR activity. Finally, we also discovered a novel Type V TA system that is activated by MqsR, the first TA system has been shown to regulate a second. Collectively, these studies demonstrate that MqsR‐MqsA defines a novel TA system and are ideal targets for developing a new class of antibiotics that inhibit the persister state.This work was supported by an NSF‐CAREER award (MCB0952550) to RP.